1. Field of the Invention
This invention relates generally to nuclear reactors and more particularly to nuclear reactor safety and the elimination of health hazards.
2. Description of the Prior Art
A nuclear reactor is designed and operated for the purpose of initiating and maintaining a nuclear fission chain reaction in a fissile material for the generation of heat for power purposes. In the type of nuclear reactor described herein, fissile materials such as plutonium-239 and uranium-238, are contained within fuel rods or elements. A plurality of fuel elements comprise a nuclear core which is structurally supported within a hermetically sealed pressure vessel. A reactor coolant, such as liquid sodium, is circulated into the pressure vessel and through the nuclear core where the heat generated by nuclear fission is transferred from the fuel elements to the reactor coolant. The heated reactor coolant exits from the pressure vessel and flows to a heat exchanger where the heat previously acquired is transferred to another flow system coupled in sealing arrangement with the heat exchanger. The cooled liquid sodium exits from the heat exchanger and flows to a pump which again circulates the reactor coolant into the pressure vessel, repeating the described flow cycle. The system comprising the nuclear core, pressure vessel, heat exchanger, circulating pump, and the connecting piping is commonly referred to as the primary system.
The heat transferred from the reactor coolant on flowing through the heat exchanger is eventually transformed into steam which is converted into electrical energy by means of a conventional steam generator, steam turbines, and electrical generator apparatus. This system, by which the heat is converted into electricity, is known as the secondary system.
During operation of the nuclear reactor, fission gas and solids are produced by fission of the fissile nuclear fuel. These fission products generally contain radioactive nuclides including iodine-131 and iodine-125. Should these radioactive nuclides be released to the reactor coolant in the primary system, a biological hazard and a reactor safety problem may exist.
The health and safety problem is caused mainly by deposition of the radioactive iodine onto all surfaces in the primary system with which the contaminated reactor coolant comes in contact and subsequent exposure of personnel to the contaminated surfaces. This includes the surfaces of such apparatus as the reactor pressure vessel, the pressure vessel closure head, main coolant circulating pumps, heat exchangers, connecting piping, valves and other like apparatus. During normal operation, the health and safety problem does not exist because personnel do not expose themselves to the primary system components contaminated with radioactive iodine. During such operations as reactor refueling, reactor maintenance and primary system repairs, personnel will probably expose themselves to the radioactive components and the health hazard and reactor safety problems can exist.
As stated above, the health and safety problem is initially caused by release of fission products from the fuel elements or rods to the reactor coolant. Although the extent of the release of these fission products depends on the type of fuel rod used in the reactor, some fission product release can occur regardless of the type of fuel rod used. Unvented fuel rods comprising sealed cladding tubes containing fuel pellets are designed to keep the fission products contained within the fuel rod. Unvented fuel rods however have the disadvantage of limited fuel life due to the build-up of fission products which increases as a function of the reactor operation; and, the gas pressure built up within the sealed fuel rods, by the fission gases, places undue design limitations on the fuel cladding. Further, there is the possibility of rupture of one or more cladding tubes during reactor operation which results in releasing fission products to the reactor coolant. Vented fuel rods comprising fuel pellets contained within unsealed cladding tubes eliminates these problems; but, venting of the rods allows the fission products to be released directly to the reactor coolant. Thus, with either type of fuel rod it is possible for the reactor coolant to become contaminated with radioactive iodine-131 and iodine-125.
In the prior art, removal of the radioactive iodine contamination from the reactor coolant has been only partially effective. One reason for this is that iodine removal was accomplished as a by-product during removal of oxygen contamination by a cold trapping technique. Cold trapping is a process which operates by lowering the temperature of a contaminated liquid thereby reducing the solubility of the contaminant in the liquid and then precipitating the contaminant, such as oxygen, out of solution. It has been previously found that this oxygen removal process resulted in removal of approximately 50% of the iodine-131 present in the solution.
Another technique which has been employed to remove radioactive iodine involves adding hydrogen to contaminated sodium and then precipitating sodium hydride out of solution. The precipitated sodium hydride has been shown to contain the radioactive iodine isotopes as an impurity within the precipitant. Although this art does teach the effective removal of radioactive iodine from a reactor coolant, it suffers from the complexity of having to add gaseous hydrogen to a liquid. Another disadvantage of the prior art is the difficulty of determining if all or essentially all of the radioactive iodine isotopes have been removed because of the difficulty of determining the extent of the iodine contamination. Therefore, in the prior art, the biological hazard and the reactor safety problem of iodine contamination of the reactor coolant is not adequately eliminated, a relatively complex solution has been employed, or the degree of certainty of removal of the iodine is inadequate.